The invention relates to a process for improving the determination of the wheel radius ratios in a vehicle. In particular, the invention relates to the improved determination of wheel radius ratios for the purpose of detecting an unacceptable tire pressure.
A prior art process for the detection of unacceptable tire pressure is disclosed in German patent document 43 27 492 C1 (U.S. Pat. No. 5,583,483), incorporated herein by reference.
In the prior art, vehicles having an anti-lock braking system typically use a rotational-speed sensing device. This sensing device is usually installed in the proximity of each wheel, and consists of a toothed magnet wheel with an electromagnetic clocking device attached to the wheel suspension. The rotational-speed of the wheel can then be determined directly by means of this sensing device, and the wheel speed can be determined indirectly from the wheel radius.
In evaluating the rotational-speed signals of a vehicle""s wheels within the framework of regulating or warning functions, the rotational-speed signals are compared with each other to detect e.g., a slip regulation, or a tire pressure warning. However, when the vehicle is traveling around a curve, there is a difference between rotational-speed signals that should not be used to trigger the regulating or warning functions. This difference in rotational-speed signals, due to traveling around a curve, is not easily distinguishable from the difference in rotational-speed signals caused by wheel slip, or by changes in tire pressure. In the prior art, this problem is typically circumvented by setting a predetermined curve travel limit value, which effectively removes the curve-related rotational speed factor from the process.
Therefore, it is an object of the present invention to improve the process for determining the ratios between the radii of vehicle wheels (the wheel radius ratio), so that they can be used during curve travel, as well as normal travel, without any of the prior art limitations.
This object is attained by the inventive process, as follows:
a) measuring a rotational-speed of each wheel of the vehicle,
b) calculating an angular velocity for each wheel of the vehicle, based on the rotational-speed measurements,
c) measuring a vehicle movement signal,
d) calculating a yawing rate for the vehicle, based on the vehicle movement signal measurement,
e) calculating a drive slip for each side of the vehicle, based on the corresponding angular velocities and the yawing rate, where the drive slip calculation uses the yawing rate to compensate for curve travel effects, and
f) determining the wheel radius ratios, based on the calculated drive slip signal and a driving force signal received from the vehicle.
The inventive process requires only one vehicle movement signal for the calculation of curve travel compensation, and this movement signal can be obtained from a sensor, which is typically present in vehicles having travel dynamics regulating systems. As such, curve travel compensation can be achieved through computation alone, and can be applied in practice without additional cost.
A yawing rate sensor is preferably used to determine the vehicle movement signal. In an alternative embodiment of the invention, however, a steering angle sensor and/or a transverse acceleration sensor can be used. The yawing rate of a vehicle can also be determined in accordance with German patent document 198 17 686 A1 (U.S. Pat. No. 6,092,415) (incorporated herein by reference), by using the signals from these sensors and the vehicle speed. If several of the above-mentioned sensors are available, their output signals can be used for a mutual plausibility check.
Once the wheel radius ratios are determined accurately, including curve travel compensation, they are monitored continuously, so that a predetermined amount of change in ratios attributable to one of the wheels indicates an unacceptable tire pressure in that wheel.